Arrangement for controlling a hydraulically operated friction disc clutch with diaphragm spring

ABSTRACT

The invention relates to an arrangement for automatic control of a hydraulically operated friction disc clutch with diaphragm spring. The hydraulic operating unit comprises a hydraulic cylinder which, by acting on the diaphragm spring, disengages the friction disc clutch. In order to obtain a hydraulic operating unit in which the regulating pressure is proportional to the disengagement degree of the clutch, the hydraulic cylinder comprises a proportionality-forming spring which acts against the hydraulic cylinder piston during the final part of the disengagement stroke of the piston. In a system with master cylinder and slave cylinder, where the slave cylinder acts on the diaphragm spring for disengagement of the friction disc clutch, the master cylinder is also provided with a pressure supply, compensating for wear of the discs, via a hydraulic channel which is closed off at a predetermined regulating pressure level. The invention permits a simplified system solution for an automated diaphragm spring clutch in a vehicle, where the clutch does not require any positional sensors, but the disengagement degree is given by the hydraulic regulating device.

The invention relates to an arrangement for controlling a hydraulicallyoperated friction disk clutch that is provided with a diaphragm spring,the control being provided through an operating unit including ahydraulic piston that acts on the diaphragm spring to release theengagement of the clutch disks. The hydraulic piston is controlled by apressure medium which in turn is controlled by a control signalproportional to the extent of the disengagement of the clutch disks. Theinvention particularly relates to a spring associated with the hydraulicpiston and which comes into action to control the releasing motion ofthe diaphragm spring as the hydraulic piston shifts.

BACKGROUND OF THE INVENTION

For operating a conventional friction clutch between an engine and atransmission box in, for example, a motor vehicle, it is already knownto use hydraulic operating mechanisms with a master cylinder actuated bythe clutch pedal and a slave cylinder located at the clutch. Thiscylinder either pulls or exerts pressure on a spring-compressed discclutch for disengagement. A pressure clutch of this type is shown, forexample, in U.S. Pat. No. 3,489,257.

However, development of clutch systems, especially for motor vehicles,is moving towards more or less automated clutch functions. In automatedclutch systems an electronic control unit can regulate the entireengagement and disengagement procedure, so that the clutch at all timesacquires an optimum slippage function at each speed difference which canoccur between the input shaft and the output shaft of the clutch. Anautomated clutch can also regulate the slippage level in the clutch inorder to achieve damping of torque pulses. This technique is describedin, for example, U.S. Pat. Nos. 4,253,414 and 4,457,411.

A majority of motor vehicle clutches comprise a diaphragm springpressing the friction discs together. A solution for disengagingfriction disc clutches held together by a diaphragm spring is previouslydisclosed in GB Patent No. 2,117,076 corresponding to U.S. Pat. No.4,878,396. In this respect use is made of an electric motor of the lowoutput type, for example a windscreen wiper motor, which is able todisengage the friction disc clutch by means of a compensating springarrangement acting over the entire disengagement stroke. Thecompensating springs provide a force counter-acting the diaphragmspring, which force follows the force and deflection characteristic ofthe diaphragm spring over the entire stroke. In this way the powerrequirement of the electric motor can be kept low, by which means it ispossible, with a small and inexpensive motor, to obtain a system whichis suitable for an automated clutch function. However, the compensatingspring construction is relatively complicated, since the diaphragmspring does not have a linear force and deflection characteristic overthe entire disengagement stroke. Moreover, the system does not have thesame quick response of a hydraulic system.

The advantage of a disc clutch held together by a diaphragm spring isthat the axial dimension of the clutch can be kept low. In the case of aconventional manually effected disengagement, there is also a decreasedpower requirement at the end of the disengagement movement. In this waythe driver can hold the clutch pedal down in the bottom position for alonger period of time without becoming tired. The diaphragm springswhich are present in clutches in today's cars also afford an extremelyreliable and functional clutch at a reasonable cost and with anextremely long service life of the diaphragm spring itself.

SUMMARY OF THE INVENTION

The object of the present invention is to provide for an automatedhydraulic operating of a friction disc clutch held together with adiaphragm spring and with a simple hydraulic piston cylinder. In orderto automate a hydraulically operated diaphragm spring clutch, the systemcan be modified so that, at each specific operating pressure, the clutchacquires a predetermined disengagement degree. This eliminates the needfor expensive positional indicators in the clutch. To this end, thearrangement of the invention includes a spring which cooperates with thehydraulic piston to oppose movement of the hydraulic piston to thesecond position after the hydraulic piston has moved from the firstposition to an intermediate position. The spring applies an increasingforce to oppose movement of the hydraulic piston to the second position,which imposes an increasing force requirement while the diaphragm springis decreasing its engaging force on the clutch disks. The spring isdimensioned and adapted so that an essentially proportionally increasingforce requirement is obtained over essentially the whole disengagementstroke of the hydraulic piston, at least from the intermediate positionto the second end position of the hydraulic piston.

The invention can be used both in systems with an operating cylinderacting directly on the diaphragm spring, and in systems withmaster-slave cylinder. Integrating the proportionality-forming spring ina hydraulic cylinder means that this spring is given a well-protectedposition, so that operational reliability and functioning are ensured.

In a system with master and slave cylinder, a proportionality-formingspring can be integrated in the master cylinder or in the slavecylinder, or in both these cylinders. This makes it possible, whenconstructing the system, to position the proportionality-forming springwhere there is space in the clutch system and car model in question,either in a master cylinder mounted remote from the clutch or in theslave cylinder close to the clutch.

In an advantageous embodiment of a master-slave cylinder construction inthe arrangement according to the invention, the master cylinder can bedesigned so that at the same time a clearance adjustment of thedisengagement mechanism is obtained as the discs in the clutch are worn,and a constant prestressing of the slave cylinder against the diaphragmspring is made possible. To this end, the arrangement is characterizedby the provision of a restricted inlet or channel connected from thepressure source into the bore of the master cylinder at the outlet sideof the master cylinder which is the side of the master cylindercommunicating with the inlet side of the slave cylinder. The restrictedinlet communicates into the master cylinder bore only when the piston ofthe master cylinder is in its first, neutral end position. There is aradial seal around the piston in the master cylinder which seals thebore of that cylinder and the seal is placed to enable pressure inletinto the master cylinder through the restricted inlet when the masterpiston is in its first position.

In a further advantageous embodiment, the master cylinder piston isdesigned in such a way that, at a predetermined pressure level, aproportional valve guarantees closure of a clearance-adjusting andprestress-forming channel. The design of the master cylinder piston alsoprovides for a power increase, by reason of which a smaller and lessexpensive hydraulic pump can be chosen. To this end, the arrangement issuch that the master cylinder is stepped in the axial direction and thepiston in the master cylinder is correspondingly shaped so that it has alarger surface area at its end toward the pressure source inlet side ofthe master cylinder and a smaller surface area at its end toward theoutlet side of the master cylinder. The spring is a return springarranged in the master cylinder for forcing the piston of the mastercylinder toward its first end position.

The essential feature according to the invention provides for anextremely simple and inexpensive hydraulic cylinder construction forproportional control of a clutch with essentially only one movablepiston which acts directly on the diaphragm spring with springs arrangedin the hydraulic cylinder and acting on the piston.

The restricted inlet or channel into the master cylinder, combined withthe stepped shape of the piston of the master cylinder and coupled withthe spring for the hydraulic piston and particularly such spring at thepiston of the master cylinder also permit clearance adjustment,prestressing of the clutch and a reliable disengagement function atpredetermined pressure levels, without the hydraulic cylinderconstruction departing from its simple basic design.

A hydraulic cylinder designed as a master cylinder according to theinvention also makes it possible for a slave cylinder construction,which is incorporated in a manual system, to be used in an automatedclutch with proportionately increasing power requirements over thedisengagement stroke. The same drive-line layout can thus be used inboth manual and automated clutch systems. This is of particularly greatimportance in motor vehicles where the components in the drive-lineengine, clutch and transmission box - have a limited space in the enginebay. The master cylinder construction can be positioned wherever desiredin the available space in the engine bay and not necessarily close tothe clutch.

Other characterizing features will emerge from the patent claims and thefollowing description of an embodiment of the arrangement according tothe invention. In the description, reference is made to the attachedfigures.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a master cylinder according to the invention in partialaxial section with the piston in a neutral position, where the clutch isin engagement.

FIG. 2 shows the master cylinder with only the master cylinder housing,the springs and the sealing rings of the piston in section. Here, themaster cylinder piston is in an intermediate position, in which thepiston is beginning to engage against a proportionality-forming spring,in which position the clutch is partially disengaged.

FIG. 3 shows the master cylinder in the same section as in FIG. 2, withthe piston in an operational end position in which the clutch iscompletely disengaged.

FIG. 4 shows a basic system design, in which the master cylinderaccording to the invention is incorporated.

FIG. 5 shows the effect of the proportionality-forming spring on thenecessary regulating pressure on the master cylinder.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

FIGS. 1-3 show a master cylinder 1 according to the invention in threedifferent operational positions.

The master cylinder 1 has a housing 2 made in one piece with an open endclosed by means of a sealed end cap 3. The housing is advantageouslybored out from the end in two stages in the axial direction, with afirst outer bore which is close to the end and has a greater diameterthan a second inner bore. The bores accommodate a piston 4 which isaxially movable between two end positions shown in FIGS. 1 and 3.

The piston is step-shaped corresponding to the bores, with a larger partdirected towards the end and running in the first bore, and a smallerpart directed away from the end and running in the second bore. Thelarger part of the piston 4 is sealed off, by means of a radial seal 11,against the wall of the first bore, while the smaller part of the piston4 is sealed off, by means of two radial seals 10a, 10b, against the wallof the second bore. At the bottom of the smaller bore there is also aspring-centering device 20. On the spring-centering device there is areturn spring 14 which is stressed against the piston 4 so that, in theneutral position according to FIG. 1, the piston is pressed against astop lug 18 formed on the end cap 3. The return spring 14 is centered onan outer part 22 of the spring-centering device 20 and an axial bore 30in the piston 4. The return spring 14 has a low spring constant and issimply designed to ensure that the piston returns completely to theneutral end position where it bears against the stop lug 18 and wherethe piston is not acted upon by any regulating pressure.

Also arranged on the spring-centering device 20 is a powerfulproportionality-forming spring 13, which is hereinafter referred to asthe main spring, with a significantly higher spring constant than thereturn spring 14. The main spring 13 is centered on a large base part 21of the device 20 and lies coaxially outside the return spring 14. Thelength of the main spring 13 is designed so that it comes into springengagement between the piston 4 and the base of the smaller bore onlywhen the piston 4 has left its neutral position according to FIG. 1 andhas reached an intermediate position as shown in FIG. 2. In this way,the main spring 13 provides a counter-directed and increasing forceduring the continued movement of the piston towards the operating endposition shown in FIG. 3. The force arising from the main spring 13 fromthe intermediate position is added to the force from a diaphragm springincorporated in the clutch, which forces are to be overcome by theregulating pressure of the master cylinder for the purpose of completedisengagement.

The position of the master cylinder piston 4 is controlled by means of ahydraulic pressure acting on the top side 31 of the piston in FIGS. 1-3.The regulating pressure is supplied to the inlet chamber 16 via achannel 5 in the master cylinder housing 2, by means of which thehydraulic pressure prevailing in the inlet chamber 16 provides a forcewhich acts on the inlet side 31 of the piston and will push the piston 4to the left towards the operating end position shown in FIG. 3. With asufficiently high regulating pressure in the inlet chamber 16, thepiston 4 moves from the neutral position in FIG. 1 to the operating endposition shown in FIG. 3, while a hydraulic fluid volume situated in thespace 17 is forced out through an outlet channel 6 and acts upon a slavecylinder for disengaging a diaphragm spring clutch.

The master cylinder housing 2 also comprises a restricted inlet channel7 which opens out in the wall of the second bore. The restricted channel7 is only open to the space 17 when the piston 4 is in its neutral endposition according to FIG. 1. In this end position the radial seal 10aleaves the channel opening free with respect to a bevel 9 on the piston4. By means of moderate pressure of a hydraulic fluid supplied to thechannel 7, the oil volume between the master cylinder and the slavecylinder can be automatically controlled in the neutral end position.This compensates for wearing of the discs in the diaphragm springclutch, which wear otherwise leads to the slave cylinder changing itsneutral position. Moderate pressure also provides for a certainpre-stressing of the slave cylinder against the diaphragm spring in theneutral end position. The pre-stressing eliminates any possiblelooseness between the master cylinder movement and the subsequentdiaphragm spring actuation.

The space 15 to the other side of the pressurized space 16 of the piston4 is drained to the atmosphere via a channel 8. This prevents anyleakage flow past the radial seal 11 of the piston from forming any backpressure. The smaller part of the piston 4 also comprises a secondradial seal 10b which counteracts any leakage of hydraulic fluid fromthe channel 7 to the drained space 15 when the piston is not in itsneutral end position.

The master cylinder 1 is incorporated in a hydraulically operated clutchsystem, which is shown diagrammatically in FIG. 4. A pump 40 withpressure accumulator 42 provides a required system pressure. The suctionside of the pump takes hydraulic fluid from a tank 41 via the pipeline50 and provides a hydraulic pressure via the pipeline 51 to a pressureaccumulator 42. Via the pipeline 52 a proportional valve 43 can thenprovide a reduced system pressure via the pipeline 53 to the mastercylinder 1. An electronic control unit 44 controls the level of thehydraulic pressure to the master cylinder by emitting a control signalvia the electric line 47, in which respect the signal level isproportional to the desired regulating pressure on the master cylinder 1and corresponding degree of disengagement in a conventional frictionclutch 60. The level of the control signal is regulated by the controlunit 44 as a function of input signals via the electric lines 45. Theseinput signals can represent various vehicle parameters such as the speedof the engine and the input shaft on the transmission box. By detectingthese speeds, the slip function of the clutch can be controlled. Inautomatic transmission systems an input signal can also come from a gearshift sensor which indicates that the driver has ordered transmission,after which the control unit quickly disengages the clutch by means of ahigh signal being emitted via the electric line 47. Once the new gearhas been engaged, acknowledgment contacts on the transmission box givean input signal regarding engagement. A number of other input signalsrepresenting parameters such as, for example, brake pedal actuation andaccelerator position, can be used by the control unit for operating theclutch.

The master cylinder 1 receives the regulating pressure proportional tothe control signal 47 via the branch lines 53a and 53b which areconnected to inlets 5 and 7, respectively, on the master cylinderhousing. The outlet channel 6 of the master cylinder 1 is in turnconnected to a slave cylinder 61 via the pressure line 54. In this casethe slave cylinder is mounted centrally around the input shaft 67 of thetransmission box with an annular slave cylinder piston 62. The slavecylinder piston 62 acts via a clutch collar 63 on a diaphragm spring 64incorporated in the clutch. The clutch in FIG. 4 is of the pressuretype, in which disengagement is achieved when the slave cylinder presseson the diaphragm spring 64 and the diaphragm spring turns around theabutment 69 which is shown only diagrammatically. The clamping force ofthe diaphragm spring 64 on the pressure plate 65 and the friction discs66 on the boss part 70 of the axle 67 is released upon disengagement, bywhich means the power transmission between the flywheel 68 of the engineand the input shaft 67 of the transmission box ceases.

Drained hydraulic fluid is returned from the channel 8 via the line 55to the tank 41. Hydraulic fluid is also returned via the line 56 fromthe master cylinder 1 to the tank 41 when the proportional valve 43shuts down the supply from the accumulator 42 and the piston returns tothe neutral end position.

When the two inlets 5 and 7 of the master cylinder are pressurized bythe same regulating pressure from the proportional valve 43, the mastercylinder piston 4 begins to leave the neutral end position according toFIG. 1 only when the net pressure force exceeds the force of the returnspring 14 on the piston 4. The net pressure force is given by the areadifference between the larger and smaller parts of the piston 4multiplied by the regulating pressure. Consequently, the piston 4 shutsoff the channel 7 at a predetermined pressure level given by the returnspring force and the area difference. At lower pressure levels, thepiston does not move from the neutral end position, but the slavecylinder is given a certain pre-stressing against the diaphragm springby means of the hydraulic pressure in the restricted channel 7.

FIG. 5 shows the effect of the proportionality-forming main spring 13 onthe necessary regulating pressure on the master cylinder for disengagingthe diaphragm spring clutch. The curve 80 is, for reasons of clarity,displaced somewhat in the negative x-direction. The diagram shows theregulating pressure P as a function of the disengagement degree X. Thecurve 81 shows how the necessary regulating pressure P for disengagementof a diaphragm spring clutch increases to a disengagement degree X whichcorresponds to a master cylinder piston movement of approximately 4 mm.However, as the disengagement degree continues to increase, thenecessary regulating pressure decreases. This means that an appliedregulating pressure can correspond to two disengagement positions.Automation of such a system results in the disadvantage that the systemmust comprise positional sensors on, for example, the slave cylinderpiston. The positional sensors then provide information to the controlunit on the disengagement degree of the clutch.

The curve 80 shows instead how the necessary regulating pressureincreases with a proportionality-forming spring according to the presentinvention. When the main spring begins to cooperate with the mastercylinder piston in the intermediate position according to FIG. 2, theregulating pressure requirement for the chamber 16 increases from theintermediate position. The increase in the necessary regulating pressurewhich the main spring demands is shown by the scored area A in FIG. 5.The intermediate position is directly defined by the spring constant ofthe proportionality-forming spring 13 and the diaphragm springcharacteristic. In order for the clutch to obtain a given disengagementposition for each regulating pressure, the intermediate position in thefirst place must occur before the diaphragm spring force begins toreduce for increasing disengagement degree. The spring constant andlength of the spring member 13 must then be adapted to the diaphragmspring characteristic so that the intermediate position occurs so earlyin the disengagement stroke that the spring member 13 has time todevelop a force which exceeds the force reduction of the diaphragmspring at the end of the disengagement stroke. This means that a springmember 13 with low spring constant must act on the master cylinderpiston earlier during the disengagement stroke, and with a high springconstant the intermediate position where the spring acts on the pistonis postponed. Thus, with a proportionality-forming spring in the mastercylinder according to the invention, a hydraulically-operated diaphragmspring clutch is obtained in which the regulating pressure level isproportional to the disengagement degree. The requirement for separatepositional sensors on the master or slave cylinder is thus dispensedwith.

The invention should not be regarded as being limited to the embodimentdescribed, but can be modified in a number of embodiments within thescope of the subsequent patent claims.

We claim:
 1. An arrangement for controlling a hydraulically operateddisk clutch, wherein the clutch includes clutch disks and a diaphragmspring for normally biasing the clutch disks into engagement, thediaphragm spring being operable against its normal bias to graduallydisengage the clutch disks, the arrangement comprising:hydraulic meansfor acting on the diaphragm spring, the hydraulic means having a movingelement operable between a first end position at which the hydraulicmeans enables the diaphragm spring to bias the clutch disks intoengagement and a second opposite end position where the diaphragm springis operated against its normal bias to release the engagement of theclutch disks; the hydraulic means having an inlet side which ispressurized for moving the moving element from the first position towardthe second position thereof; a proportioning valve adapted to receivepressure from a pressure source, the proportioning valve being coupledwith the inlet side of the hydraulic means; a control unit coupled tothe proportioning valve for delivering a control signal to theproportioning valve, said signal being proportional to the extent ofdisengagement of the clutch disks, said signal controlling the hydraulicpressure supplied to the inlet side of the hydraulic means so that thepressure at the inlet side of the hydraulic means is proportional tovariations in the signal from the control unit; a second springsupported at the hydraulic means and positioned so as not to restrainmovement of the moving element from the first position thereof to anintermediate position between the first and second positions, and thesecond spring being further positioned to be engaged by the movingelement following the movement of the moving element from the first tothe intermediate positions; upon engaging the moving element, the secondspring being adapted for applying increasing force opposing movement ofthe moving element to the second position and imposing an increasingforce requirement while the diaphragm spring is decreasing its engagingforce upon the clutch disks, and the second spring being dimentioned andadapted such that a substantially increasing force requirement isobtained over the disengagement stroke of the moving element, at leastfrom the intermediate position to the second end position of the movingelement.
 2. The arrangement of claim 1, wherein the hydraulic meanscomprises:a master cylinder with a bore through it, a master pistonaxially movable in the master cylinder bore between a first end positioncorresponding to the first end position of the moving element and anopposite second end position corresponding to the second end position ofthe moving element, the master cylinder bore having a first inlet sideat one side of the master piston and a first outlet side at the otherside of the master piston; and further comprising: a slave cylinder witha bore through it, a slave piston axially movable in the slave cylinderbore between a slave first end position obtained when the master pistonis in its first end position and an opposite slave second end positionobtained when the master piston is in its second end position; the slavecylinder having a second inlet side at one side of the slave piston; oneof the first and second inlet sides being the inlet side of thehydraulic means; the slave piston being coupled to the diaphragm springfor moving the diaphragm spring; a pressure line connecting the firstoutlet side of the master cylinder with the second inlet side of theslave cylinder for hydrostatic transmission of the movement of themaster piston through the pressure line to the slave piston, forcorrespondingly moving the slave piston upon actuation of theproportioning valve, and for causing the slave piston to act on thediaphragm spring, upon movement of the slave piston toward the secondposition thereof, thereby disengaging the clutch disks; the secondspring being accommodated in at least one of the master cylinder and theslave cylinder and the second spring being shaped and positioned forcooperating with the respective one of the master piston and the slavepiston in the respective cylinder in which the second spring isaccommodated when the respective piston has reached the intermediateposition in moving from the first to the second end positions thereof.3. The arrangement of claim 2, wherein the master cylinder has arestricted inlet connected to the pressure source and connected into themaster cylinder bore at a location such that hydraulic pressure isdelivered through the restricted inlet into the first outlet side of themaster cylinder when the master piston is in the first end positionthereof; the master piston has a radial seal on it which seals the firstinlet side from the first outlet side of the master cylinder, and therestricted inlet and the radial seal on the master piston being soplaced that the restricted inlet communicates into the first outlet sideof the master cylinder only when the master piston is in the firstposition thereof, and upon the master piston moving off the firstposition and toward the intermediate and the second positions thereof,the master piston being shaped for blocking entrance into the mastercylinder from the restricted inlet.
 4. The arrangement of claim 3,wherein the master cylinder is stepped in the axial direction and themaster piston is correspondingly shaped such that the master piston hasa larger surface area at the first inlet side and has a smaller surfacearea at the first outlet side and a return spring is arranged in therespective cylinder for biasing the respective piston toward the firstposition thereof after the respective piston has shifted from the firstposition to the intermediate position; the second spring in therespective cylinder is so shaped and of such length, and the diaphragmspring and the slave piston in engagement therewith and the masterpiston are all so shaped and positioned that the intermediate positionof the second respective piston, where the spring is first engaged bythe respective piston, occurs at a predetermined disengagement positionof the diaphragm spring and the clutch disks which is before thediaphragm spring has moved to fully disengage the clutch disks, so thatfurther movement of the respective piston from the intermediate positionthereof toward the second position thereof causes the diaphragm springto disengage increasingly the clutch disks and causes the spring forceof the diaphragm spring to decrease.
 5. The arrangement of claim 1,wherein the hydraulic means comprisesa master cylinder with a borethrough it, the moving element comprising a master piston axiallymovable in the master cylinder bore between the first end position andthe opposite second end position, the master cylinder bore having afirst inlet side at one side of the master piston and a first outletside at the other side of the master piston; and further comprising: aslave cylinder with a bore through it and an axially movable slavepiston in the slave cylinder and movable between a first end positionobtained when the master piston is in its first end position and anopposite second end position obtained when the master piston is in itssecond end position; the slave cylinder having a second inlet side atone side of the slave piston, the first inlet side being the inlet sideof the hydraulic means; the slave piston being coupled to the diaphragmspring for moving the diaphragm spring; a pressure line connecting thefirst outlet side of the master cylinder with the second inlet side ofthe slave cylinder for hydrostatic transmission of the movement of themaster piston through the pressure line to the slave piston, forcorrespondingly moving the slave piston upon actuation of theproportioning valve, and for causing the slave piston to act on thediaphragm spring, upon movement of the slave piston toward the secondposition thereof, thereby disengaging the clutch disks; the secondspring being accommodated in the master cylinder and the second springthereby cooperating with the master piston when the master piston hasreached the intermediate position in moving from the first to the secondend positions thereof.
 6. The arrangement of claim 5, wherein the mastercylinder has a restricted inlet connected to the pressure source andconnected into the master cylinder bore at a location such thathydraulic pressure is delivered through the restricted inlet into thefirst outlet side of the master cylinder when the master piston is inthe first end position thereof.
 7. The arrangement of claim 6, whereinthe master piston has a radial seal on it which seals the first inletside from the first outlet side of the master cylinder, and therestricted inlet and the radial seal on the master piston being soplaced that the restricted inlet communicates into the first outlet sideof the master cylinder only when the master piston is in the firstposition thereof, and upon the master piston moving off the firstposition and toward the intermediate and the second positions thereof,the master piston being shaped for blocking entrance into the mastercylinder from the restricted inlet.
 8. The arrangement of claim 7,wherein the master cylinder is stepped in the axial direction and themaster piston is correspondingly shaped such that the master piston hasa larger surface area at the first inlet side and has a smaller surfacearea at the first outlet side and a return spring is arranged in themaster cylinder for biasing the master piston toward the first positionthereof after the master piston has shifted from the first position tothe intermediate position.
 9. The arrangement of claim 5, wherein thesecond spring in the master cylinder is so shaped and of such length,and the diaphragm spring and the slave piston in engagement therewithand the master piston are all so shaped and positioned that theintermediate position of the master piston, where the second spring isfirst engaged by the master piston, occurs at a predetermineddisengagement position of the diaphragm spring and the clutch diskswhich is before the diaphragm spring has moved to disengage fully theclutch disks, so that further movement of the master piston from theintermediate position thereof toward the second position thereof causesthe diaphragm spring to disengage increasingly the clutch disks andcauses the spring force of the diaphragm spring to decrease.
 10. Thearrangement of claim 9, wherein the intermediate position of the masterpiston is related to the spring constant of the second spring, such thatat a lower spring constant, the intermediate position of the masterpiston is relatively closer to the first position of the master piston,and at a higher spring constant, the intermediate position of the masterpiston is relatively closer to the second end position of the masterpiston, enabling the force that is exerted by the second spring againstthe movement of the master piston from the intermediate position towardthe second position to develop great enough to compensate for the forcereduction of the diaphragm spring on the clutch disks toward the end ofthe disengagement stroke, which end is at the second position of themaster piston.
 11. The arrangement of claim 8, wherein the master pistonhas a second radial seal on it which is axially spaced apart from thefirst mentioned radial seal on the master piston, and both radial sealssealing the master piston in the master cylinder; the first of theradial seals being located further toward the first outlet side of themaster cylinder and being so located that the restricted inlet maydeliver hydraulic fluid to the master cylinder first outlet side whenthe master piston is at the first end position, and the first and secondradial seals being respectively so located that with the master pistonoff the first position, the first radial seal toward the first outletside of the master cylinder blocks passage of hydraulic pressure fromthe restricted inlet to the first outlet side; and the second radialseal, which is axially closer to the first inlet side of the mastercylinder, blocks hydraulic pressure from the restricted inlet toward thefirst inlet side of the master cylinder.
 12. The arrangement of claim11, wherein the master cylinder and master piston are respectively soshaped as to define an additional chamber in the master cylinder at themaster piston between the larger surface area and the smaller surfacearea of the master piston; a third radial seal around the master pistonbetween the second seal and the first inlet side of the master cylinderfor sealing the additional chamber in the master cylinder, and a drainconnection from the additional chamber to atmospheric pressure, therebyavoiding buildup of pressure in the additional chamber during movementof the master piston from the first position toward the second positionthereof.
 13. The arrangement of claim 7, wherein the master piston has asecond radial seal on it which is axially spaced apart from the firstmentioned radial seal on the master piston, and both radial sealssealing the master piston in the master cylinder; with the first of theradial seals being located further toward the first outlet side of themaster cylinder and being so located that the restricted inlet maydeliver hydraulic fluid to the master cylinder first outlet side whenthe master piston is at the first end position, and the first and secondradial seals being respectively so located that with the master pistonoff the first position, the first radial seal toward the first outletside of the master cylinder blocks passage of hydraulic pressure fromthe restricted inlet to the first outlet side; and the second radialseal, which is axially closer to the first inlet side of the mastercylinder, blocks hydraulic pressure from the restricted inlet toward thefirst inlet side of the master cylinder.
 14. The arrangement of claim 1,wherein the second spring at the hydraulic means is so shaped and ofsuch length, and the diaphragm spring and the moving element inengagement therewith are all so shaped and positioned that there is anintermediate position of the moving element, where the second spring isfirst engaged by the moving element, which occurs at a predetermineddisengagement position of the diaphragm spring and the clutch disks, andwhich is before the diaphragm spring has moved to disengage fully theclutch disks so that further movement of the moving element from theintermediate position thereof toward the second position thereof causesthe diaphragm spring to disengage increasingly the clutch disks andcauses the spring force of the diaphragm spring to decrease.
 15. Thearrangement of claim 1, wherein the substantially increasing forcerequirement comprises a substantially proportionally increasing forcerequirement over substantially the entire disengagement stroke of themoving element.
 16. Apparatus for controlling a clutch, wherein theclutch includes clutch disks and a first spring means for normallybiasing the clutch disks into engagement, the first spring means havinga normal bias and being operable against the normal bias to disengagethe clutch disks, the apparatus comprising:hydraulic means forgenerating a force acting on the first spring means, the hydraulic meanshaving a moving element movable between a first end position at whichthe hydraulic means enables the first spring means to bias the clutchdisks into engagement and a second opposite end position where the firstspring means is operated against the normal bias to release theengagement of the clutch disks; the hydraulic means having an inlet sideadapted to be pressurized for moving the moving element from the firstposition toward the second position thereof; a second spring meanssupported at the hydraulic means and positioned so as to not restrainmovement of the moving element from the first position thereof to anintermediate position between the first and second positions, the secondspring means being further positioned to be engaged by the movingelement following movement of the moving element from the first to theintermediate position; the second spring means being adapted to applyincreasing force opposing movement of the moving element when the secondspring means engages the moving element at the intermediate position,thereby imposing an increasing force requirement on the moving elementwhen the moving element moves from the intermediate position to thesecond position and the first spring means is decreasing its engagingforce upon the clutch disks, said second spring means being dimensionedand adapted such that a substantially increasing force requirement isobtained over the disengagement stroke of the moving element, at leastfrom the intermediate position to the second end position of the movingelement.
 17. The apparatus of claim 16, wherein the hydraulic meanscomprises a master cylinder receiving said moving element, said movingelement comprising a master piston axially movable in the mastercylinder between the first end position and the opposite second endposition, the master cylinder having said first inlet side at one sideof the master piston and a first outlet side at the other side of themaster piston; and further comprising:a slave cylinder receiving anaxially movable slave piston movable between a first end position,obtained when the master piston is in its first end position, and anopposite second end position obtained when the master piston is in itssecond end position, the slave cylinder having a second inlet side atone side of the slave piston coupled to the first outlet side of themaster cylinder; the slave piston being coupled to the first springmeans for moving the first spring means; said slave piston being movedin said slave cylinder by hydrostatic transmission of fluid through apressure line coupling the first outlet side of the master cylinder withthe second inlet side of the slave cylinder, thereby causing said slavepiston to move toward the second position thereof thereby disengagingthe clutch disks.
 18. The apparatus of claim 17, wherein the mastercylinder has a restricted inlet adapted to be coupled to a pressuresource and coupled into the master cylinder at a location such thathydraulic pressure is delivered through the restricted inlet into thefirst outlet side of the master cylinder when the master piston is inthe first end position thereof.
 19. The apparatus of claim 18, whereinthe master piston has a radial seal thereon which seals the first inletside from the first outlet side of the master cylinder, and therestricted inlet and the radial seal on the master piston being soplaced that the restricted inlet communicates into the first outlet sideof the master cylinder only when the master piston is in the firstposition thereof, and upon the master piston moving from the firstposition and toward the intermediate and the second positions thereof,the master piston being shaped for blocking entrance into the mastercylinder from the restricted inlet.
 20. The apparatus of claim 19,wherein the master cylinder is stepped in an axial direction and themaster piston is correspondingly shaped such that the master piston hasa larger surface area at the first inlet side and has a smaller surfacearea at the first outlet side and a return spring is arranged in themaster cylinder for biasing the master piston toward the first positionthereof after the master piston has shifted from the first position tothe intermediate position.
 21. The apparatus of claim 17, wherein thesecond spring means is so shaped and of such length, and the firstspring means and the slave piston in engagement therewith and the masterpiston are all so shaped and positioned that the intermediate positionof the master piston, where the second spring means is first engaged bythe master piston, occurs at a predetermined disengagement position ofthe first spring means and the clutch disks which is before the firstspring means has moved to disengage fully the clutch disks, so thatfurther movement of the master piston from the intermediate positionthereof toward the second position thereof causes the first spring meansto disengage increasingly the clutch disks and causes the spring forceof the first spring means to decrease.
 22. The apparatus of claim 21,wherein the intermediate position of the master piston is related to thespring constant of the second spring means, such that at a lower springconstant, the intermediate position of the master piston is relativelycloser to the first position of the master piston, and at a higherspring constant, the intermediate position of the master piston isrelatively closer to the second end position of the master piston,enabling the force that is exerted by the second spring means againstthe movement of the master piston from the intermediate position towardthe second position to develop great enough to compensate for the forcereduction of the first spring means on the clutch disks toward the endof the disengagement stroke, which end is at the second position of themaster piston.
 23. The apparatus of claim 20, wherein the master pistonhas a second radial seal on it which is axially spaced apart from thefirst mentioned radial seal on the master piston, and both radial sealssealing the master piston in the master cylinder; the first of theradial seals being located further toward the first outlet side of themaster cylinder and being so located that the restricted inlet maydeliver hydraulic fluid to the master cylinder first outlet side whenthe master piston is at the first end position, and the first and secondradial seals being respectively so located that with the master pistonoff the first position, the first radial seal toward the first outletside of the master cylinder blocks passage of hydraulic pressure fromthe restricted inlet to the first outlet side; and the second radialseal, which is axially closer to the first inlet side of the mastercylinder, blocks hydraulic pressure from the restricted inlet toward thefirst inlet side of the master cylinder.
 24. The apparatus of claim 23,wherein the master cylinder and master piston are respectively so shapedas to define an additional chamber in the master cylinder at the masterpiston between the larger surface area and the smaller surface area ofthe master piston; a third radial seal around the master piston betweenthe second seal and the first inlet side of the master cylinder forsealing the additional chamber in the master cylinder, and a drainconnection from the additional chamber to atmospheric pressure, therebyavoiding buildup of pressure in the additional chamber during movementof the master piston from the first position toward the second positionthereof.
 25. The apparatus of claim 19, wherein the master piston has asecond radial seal on it which is axially spaced apart from the firstmentioned radial seal on the master piston, and both radial sealssealing the master piston in the master cylinder; with the first of theradial seals being located further toward the first outlet side of themaster cylinder and being so located that the restricted inlet maydeliver hydraulic fluid to the master cylinder first outlet side whenthe master piston is at the first end position, and the first and secondradial seals being respectively so located that with the master pistonoff the first position, the first radial seal toward the first outletside of the master cylinder blocks passage of hydraulic pressure fromthe restricted inlet to the first outlet side; and the second radialseal, which is axially closer to the first inlet side of the mastercylinder, blocks hydraulic pressure from the restricted inlet toward thefirst inlet side of the master cylinder.
 26. The apparatus of claim 16,further comprising a proportioning valve adapted to receive pressurefrom a pressure source, the proportioning valve being coupled with theinlet side of the hydraulic means, a control unit coupled to theproportioning valve for delivering a control signal to the proportioningvalve, said control signal being proportional to the extent ofdisengagement of the clutch disks, said control signal controlling thehydraulic pressure supplied to the inlet side of the hydraulic means sothat the pressure at the inlet side of the hydraulic means isproportional to variations in the control signal from the control unit.27. The apparatus of claim 16, wherein said first spring means comprisesa diaphragm spring.
 28. The apparatus of claim 16, wherein thesubstantially increasing force requirement comprises a substantiallyproportionally increasing force requirement over substantially theentire disengagement stroke of the moving element.
 29. The apparatus ofclaim 16, wherein the intermediate position is closer to the firstposition if the second spring means has a low spring constant, andcloser to the second position if the second spring means has a highspring constant.